FIG. 1 shows an exemplary wireless telecommunications network 100. The illustrative telecommunications network includes base stations 101, 102 and 103, though in operation, a telecommunications network necessarily includes many more base stations. Each of base stations 101, 102 and 103 are operable over corresponding coverage areas 104, 105 and 106. Each base station's coverage area is further divided into cells. In the illustrated network, each base station's coverage area is divided into three cells. Handset or other user equipment (UE) 109 is shown in Cell A 108. Cell A 108 is within coverage area 104 of base station 101. Base station 101 transmits to and receives transmissions from UE 109. As UE 109 moves out of Cell A 108 and into Cell B 107, UE 109 may be handed over to base station 102. Because UE 109 is synchronized with base station 101, UE 109 can employ non-synchronized random access to initiate handover to base station 102.
Non-synchronized UE 109 also employs non-synchronous random access to request allocation of up-link 111 time or frequency or code resources. If UE 109 has data ready for transmission, which may be traffic data, measurements report, tracking area update, UE 109 can transmit a random access signal on up-link 111. The random access signal notifies base station 101 that UE 109 requires up-link resources to transmit the UEs data. Base station 101 responds by transmitting to UE 109 via down-link 110, a message containing the parameters of the resources allocated for UE 109 up-link transmission along with a possible timing error correction. After receiving the resource allocation and a possible timing advance message transmitted on down-link 110 by base station 101, UE 109 optionally adjusts its transmit timing and transmits the data on up-link 111 employing the allotted resources during the prescribed time interval.
Long Term Evolution (LTE) wireless networks, also known as Evolved Universal Terrestrial Radio Access Network (E-UTRAN), are being standardized by the 3GPP working groups (WG). Orthogonal frequency division multiple access (OFDMA) and SC-FDMA (single carrier FDMA) access schemes were chosen for the down-link (DL) and up-link (UL) of E-UTRAN, respectively. User Equipments (UEs) are time and frequency multiplexed on a physical uplink shared channel (PUSCH) and time and frequency synchronization between UEs guarantees optimal intra-cell orthogonality. In UL, frequency offsets (FO) can be due to local oscillator (LO) drifts at both the UE and the Base Station, also referred to as eNodeB, but also to the UE speed translating into Doppler shift in line of sight (LOS) propagation conditions. If the large sub-carrier spacing of LTE (15 kHz) makes it robust with respect to orthogonality loss due to Doppler shift, since even high speed trains would not generate a Doppler shift exceeding one tenth of a sub-carrier, the frequency offset still has a negative impact on Block Error Rate (BLER) due to fast channel variation within a sub-frame:
Rayleigh TU channel: FO range of 0 to 300 Hz with the maximum frequency inaccuracies at eNB of 0.05 ppm and at UE of 0.1 ppm;
Additive White Gaussian Noise (AWGN) (LOS) channel: FO range of 0 to 1600 Hz with the UE speed translating into Doppler shift.
FIG. 2 shows the BLER performance degradation due to various frequency offsets with quadrature phase shift keying (QPSK) modulation and turbo coding rate of ⅓ with AWGN. FIG. 3 similarly shows the BLER performance degradation with TU-6 fading channel. There is a need to estimate and remove the frequency offset before channel estimation and demodulation. This invention is a frequency estimation method which applies directly on the de-mapped frequency sub-carriers of a UEs symbol and compares its performance with other published methods.
On top of these scenarios, the 3GPP Working Group #4 defined propagation channels specifically addressing the frequency offset estimation and compensation function which worst-case scenarios are expected to be encountered along High Speed Trains (HST) lines. FIG. 4 illustrates the frequency offset time behavior of both channels. FIG. 5 illustrates the resulting frequency variations observed within a 30 ms interval.
In an additional channel scenario, Rician fading is considered where Rician factor K=10 dB is the ratio between the dominant signal power and the variant of the other weaker signals.
It is clear from FIGS. 4 and 5 that in order to track the abrupt frequency offset variations of the HST scenarios, the eNB must permanently estimate the frequency offset of each UE. This patent application assumes the following scheme for concurrent frequency offset estimation and compensation illustrated in FIG. 6. In FIG. 6 frequency estimation is performed for a given UE during an estimation interval 610 and a frequency estimate is issued at the end of interval 610. During a given estimation interval 620 of a given UE, the frequency estimate issued by the previous interval 610 is replaced.